A. Field of the Invention
The present invention relates to the field of building design and construction, and more particularly to energy dissipating devices for inclusion in structural systems to protect the structure in the event of an earthquake.
B. Description of the Prior Art
The technology of seismic energy dissipation is based on the introduction of energy dissipation devices within a structural system so that seismic drift is reduced to within acceptable limits. This approach offers improved performance of the structural system to a level better than life safety, which is currently implied in the building codes. Many owners of essential and critical facilities, and of architecturally significant structures, opt for the use of this technology for achieving a performance level suitable for immediate occupancy of the structure. The approach alternatively offers a reduction of seismic drift to within limits mandated by building codes without increase in the stiffness and strength of the structural system. This does not always improve the performance level, but may either reduce the cost of new structures or allow for cost-effective rehabilitation of existing structures.
Engineers are familiar with and have so far exclusively used diagonal (FIG. 1) and chevron (FIG. 2) brace configurations for the delivery of forces from energy dissipation devices to the structural frame. Such configurations have disadvantages that inhibit the use of energy dissipation systems. More specifically, they typically occupy an entire bay in a frame and thus interfere with open space and other architectural requirements, and they are inapplicable to stiff structural frames due to small damper displacements where large damping forces are required, thus leading to expensive damper designs.
FIGS. 1 and 2 show diagonal and chevron brace configurations for the attachment of energy dissipation devices to a structural system. Detailed information on the status of this technology and its applications may be found in the monograph xe2x80x9cPassive Energy Dissipation Systems for Structural Design and Retrofitxe2x80x9d by M. C. Constantinou et al., 1998. The ineffectiveness of these configurations for stiff structural systems is well recognized and best described in the following statement from a building code:
xe2x80x83xe2x80x9cstructural systems best suited for implementation of energy dissipation devices are the moment-resisting frame and the flexible dual system, in either structural steel or reinforced concrete. The interstory response of a stiff lateral load-resisting system, such as a reinforced concrete shear wall system or a steel-braced dual system, is generally characterized by both small relative velocities and small relative displacements. As such it may not be feasible to implement supplemental energy dissipation.xe2x80x9d
Moreover, it is known that the use of energy dissipation systems has been rejected in some projects by architects concerned with interferences of the system with the desire for open space.
Energy dissipation systems installed for the improvement of the seismic performance of a structure may be ineffective in reducing wind-induced vibration. Wind-induced vibration is typically small in amplitude so that it is often ineffective, in terms of either performance or cost, to design wind energy dissipation systems within the diagonal or chevron brace configurations.
U.S. Pat. No. 5,870,863 describes a toggle linkage for incorporation into a structural frame to improve the seismic performance of the structure. The described toggle linkage comprises a first link including a damper mechanism, and second and third links that do not include damper mechanisms. All three links are coplanar, with a first end of each link being located at a different area of the structural frame. The second ends of the three links are connected proximate to each other, and a metal plate provides flexible connection between the second ends of the second and third links for allowing flexure within the plane of the linkage but prevents out-of-plane buckling of the linkage in the event of an earthquake. While the toggle linkage may be configured to perform better than the diagonal and chevron brace configurations, it also requires an entire bay for installation and, thus, it interferes with the aforementioned open space requirements.
Consequently, it is an object of the present invention to provide an energy dissipation system configuration that is applicable to stiff structural systems, or generally to systems with small structural deformations.
It is a related object of the present invention to provide an energy dissipation apparatus that can be installed in a nearly vertical configuration or at beam-to-column joints.
The energy dissipation apparatus of the present invention effectively bypasses the limitations of the diagonal and chevron brace configurations, and accordingly has an extended range of applicability.
In a preferred embodiment of the present invention, the energy dissipation apparatus comprises a scissor-jack system of braces with an energy dissipation device such as a viscous, viscoelastic, or hysteretic damper, or an active or semi-active device, connected between opposing pivot joints of the scissor-scissor jack system. The scissor jack system magnifies displacement so that energy is dissipated by the damper with a reduced requirement for damper force. The scissor jack system also magnifies the damper force through a shallow truss configuration and then delivers it to the structural frame.